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fvProv falls into a hole (instead of detecting missing type argument) (regression from -rc1 to -rc2)

    • Closed Issue created by waldmann@imn.htwk-leipzig.de @trac-waldmann

    This program has an error (T is missing an argument)

    data T a ; class C t ; instance C s => C T

    which makes 8-rc2 fall into a hole:

    /opt/ghc/ghc-8.0.0.20160204/bin/ghc -c Bug.hs
ghc: panic! (the 'impossible' happened)
  (GHC version 8.0.0.20160204 for x86_64-unknown-linux):
	fvProv falls into a hole {ap6}

    It is detected by 8-rc1:

    /opt/ghc/ghc-8.0.0.20160111/bin/ghc -c Bug.hs

Bug.hs:1:40: error:
    • Illegal instance declaration for ‘C T’
        (All instance types must be of the form (T a1 ... an)
         where a1 ... an are *distinct type variables*,
         and each type variable appears at most once in the instance head.
         Use FlexibleInstances if you want to disable this.)
    • In the instance declaration for ‘C T’

Bug.hs:1:42: error:
    • Expecting one more argument to ‘T’
      Expected a type, but ‘T’ has kind ‘* -> *’
    • In the first argument of ‘C’, namely ‘T’
      In the instance declaration for ‘C T’
    Trac metadata
    Trac field Value
    Version 8.0.1-rc2
    Type Bug
    TypeOfFailure OtherFailure
    Priority normal
    Resolution Unresolved
    Component Compiler
    Test case
    Differential revisions
    BlockedBy
    Related
    Blocking
    CC
    Operating system
    Architecture
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